// writeString writes s to w.
// If w has a WriteString method, it is invoked instead of w.Write.
func writeString(w io.Writer, s string) (n int, err error) {
    type stringWriter interface {
        WriteString(string) (n int, err error)
    }
    if sw, ok := w.(stringWriter); ok {
        return sw.WriteString(s) // avoid a copy
    }
    return w.Write([]byte(s)) // allocate temporary copy
}

// CommandLine is the default set of command-line flags, parsed from os.Args.
// The top-level functions such as BoolVar, Arg, and so on are wrappers for the
// methods of CommandLine.
var CommandLine = NewFlagSet(os.Args[0], ExitOnError)

// Parsed reports whether f.Parse has been called.
func (f *FlagSet) Parsed() bool {
	return f.parsed
}

// makeThumbnails6 makes thumbnails for each file received from the channel.
// It returns the number of bytes occupied by the files it creates.
func makeThumbnails6(filenames <-chan string) int64 {
	sizes := make(chan int64)
	var wg sync.WaitGroup // number of working goroutines
	for f := range filenames {
		wg.Add(1)
		// worker
		go func(f string) {
			defer wg.Done()
			thumb, err := thumbnail.ImageFile(f)
			if err != nil {
				log.Println(err)
				return
			}
			info, _ := os.Stat(thumb) // OK to ignore error
			sizes <- info.Size()
		}(f)
	}

	// closer
	go func() {
		wg.Wait()
		close(sizes)
	}()

	var total int64
	for size := range sizes {
		total += size
	}
	return total
}

// tokens is a counting semaphore used to
// enforce a limit of 20 concurrent requests.
var tokens = make(chan struct{}, 20)

func crawl(url string) []string {
	tokens <- struct{}{} // acquire a token

	list, err := links.Extract(url)

	<-tokens // release the token

	return list
}